A known type of brake fluid pressure generating device is disclosed in, for example, Japanese Patent Laid-Open Publication No. 9-315288. In this known brake pressure generating device, a master cylinder piston has a first master cylinder piston, a second master cylinder piston and an engage member. The first master cylinder piston and the second master cylinder piston are slidable relative to each other. When the second master cylinder piston moves toward the first master cylinder piston, the engage member engages the first master cylinder piston. One end of each of the first and the second master cylinder pistons is accommodated in a power chamber. The other ends of the first and second master cylinder pistons are accommodated in a pressure chamber. When a power fluid pressure that is generated by the supplemental fluid pressure source is sufficient, the master cylinder is driven by the regulator fluid pressure in response to the depression of a brake pedal. When a power fluid pressure that is generated by the supplemental fluid pressure source is not sufficient, the master cylinder is driven in a normal manner in response to the depression of the brake pedal. Instead of this condition, the master cylinder generates an appropriate fluid pressure.
However, this known brake fluid pressure generating device suffers from the following drawbacks and disadvantages. If the brake fluid pressure generating device experiences problems with respect to the master cylinder, the magnitude of the regulator pressure typically does not change as a result of operation of the brake pedal. As a result, the braking force that is generated by the wheel cylinders decreases.
A need thus exists for a brake pressure generating device that is able to generate a bigger magnitude regulator pressure when the master cylinder is in a disabled condition.